Configure dependabot for Docker

[hariso]

Description

This covers our Dockerfile for the Conduit Docker image. Updates for it (Go, Alpine) are relatively infrequent so I've put the schedule interval to "weekly".

This doesn't cover the Docker Compose files we have, as dependabot doesn't support them: https://github.com/dependabot/dependabot-core/issues/390

A solution that's been often mentioned in the above issue is Renovate.

Quick checks:

• [ ] I have followed the Code Guidelines.
• [ ] There is no other pull request for the same update/change.
• [ ] I have written unit tests.
• [ ] I have made sure that the PR is of reasonable size and can be easily reviewed.